Article pubs.acs.org/Macromolecules
A Low-Bandgap Conjugated Copolymer Based on Porphyrin and Dithienocoronene Diimide with Strong Two-Photon Absorption Weiyi Zhou,†,§ Feng Jin,⊥ Xuebin Huang,‡ Xuan-Ming Duan,*,⊥ and Xiaowei Zhan*,† †
Beijing National Laboratory for Molecular Sciences and CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China § Graduate University of Chinese Academy of Sciences, Beijing 100049, China ⊥ Laboratory of Organic Nanophotonics and Key Laboratory of Photochemical Convention and Functional Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Department of Chemistry, Beijing Institute of Technology, Beijing 100083, China S Supporting Information *
ABSTRACT: A new low-bandgap donor−acceptor (D−A) conjugated copolymer poly(DTCDI−POR) of planar acceptor dithienocoronene diimide (DTCDI) and strong donor porphyrin (POR) has been synthesized by Sonogashira coupling polymerization. Poly(DTCDI−POR) exhibits good thermal stability (decomposition temperature of 323 °C), strong absorption (molar extinction coefficient per repeat unit is 1.05 × 105 L mol−1 cm−1 at 468 nm in CHCl3 solution) in visible and nearinfrared region (300−900 nm), low bandgap (1.44 eV), and strong two-photon absorption (2PA) at telecommunication wavelengths with 2PA cross sections per repeat unit as high as 7809 GM at 1520 nm.
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a δ per repeat unit of 8900 GM at 1325 nm.41 Perry et al. reported that a lead-containing bis(ethynyl)porphyrin polymer exhibited a δ per repeat unit of 5200 GM at 1300 nm and of 650 GM at 1550 nm.42 We reported that a D−A conjugated copolymer of porphyrin and perylene diimide (PDI) (poly(PDI−POR), Figure 1) exhibited a δ per repeat unit of 7041 GM at 1320 nm and of 7704 GM at 1520 nm; poly(PDI− POR) is the first D−A main-chain conjugated polymer based on porphyrin.43 Recently, dithienocoronene diimide (DTCDI) was used for synthesis of organic semiconductors.44−46 Ko and co-workers reported D−A−D small molecule donors with DTCDI as acceptor, triphenylamine as donor, and bithiophene as bridge for organic solar cells, which exhibited the highest power conversion efficiency of 1.42%.44 Facchetti and co-workers reported D−A polymers of DTCDI and thiophene or bithiophene for ambipolar field-effect transistors, which exhibited electron mobilities of up to 0.30 cm2 V−1 s−1 and hole mobilities of up to 0.04 cm2 V−1 s−1.45 DTCDI has a larger size core than PDI, which could promote strong intermolecular interactions and highly ordered π-stacked structures. Moreover, DTCDI exhibits a linear monomer-linkage geometry with negligible dihedral angles and coplanar backbone in polymers,
INTRODUCTION Thanks to a wide variety of potential applications in microscopy,1−3 microfabrication,4,5 three-dimensional datastorage,6,7 optical limiting,8 and photodynamic therapy,9,10 two-photon absorption (2PA) materials have been the subject of intensive academic and commercial interest over the past 2 decades, and a number of 2PA dyes have been synthesized11−13 and investigated.14−19 Small molecule based 2PA materials have been investigated extensively, and their 2PA cross sections (δ) are as high as 105 GM (1 GM = 1 × 10−50 cm4 s−1), while the literature on 2PA polymers is significantly sparser than that for their small molecule counterparts, most of 2PA polymers exhibited 2PA cross sections below 104 GM.20−27 Current trends in device fabrication favor solution-processing. In general, polymers are more suitable for solution processing than small molecules to obtain high-quality thin films. Experimental and theoretical results on model compounds revealed that increasing the planarity and rigidity of the molecules is beneficial to 2PA enhancement.19 Owing to a planar and rigid conjugated framework with π-electrons extending in two dimensions, porphyrins (POR) represent an effective building block for 2PA materials. There have been several reports on 2PA small molecules based on porphyrin,28−40 while there are significantly sparser reports on porphyrin-based 2PA polymers. Actually, there are only three reports of porphyrin-based 2PA polymers at telecommunication wavelengths (1.3−1.55 μm).41−43 Rebane et al. reported that double-strand conjugated porphyrin ladder arrays exhibited © 2012 American Chemical Society
Received: July 21, 2012 Revised: September 17, 2012 Published: September 27, 2012 7823
dx.doi.org/10.1021/ma3015257 | Macromolecules 2012, 45, 7823−7828
Macromolecules
Article
Figure 1. Chemical structure of poly(PDI−POR) and poly(DTCDI−POR).
Scheme 1. Synthesis of Polymer Poly(DTCDI−POR)
while sterically hindered bay region in PDI causes significant polymer backbone torsion.45 In this paper, we report a low-bandgap D−A conjugated copolymer based on porphyrin and DTCDI (poly(DTCDI− POR), Figure 1). DTCDI and porphyrin units have conformational rigidity and large π-conjugation, which is common features of efficient 2PA materials. Acceptor, donor, and a polarizable π-bridge are three essential components required for strong 2PA dyes. In poly(DTCDI−POR), porphyrin is a strong π-electron donor (D), DTCDI is a strong π-electron acceptor (A), and acetylene is a π-bridge between D and A units. Polymer poly(DTCDI−POR) exhibits strong NIR absorption and large 2PA cross sections at telecommunication wavelengths (up to 7809 GM/repeat unit).
were synthesized according to our reported procedures. Stille coupling reaction between PDI-2Br and 2-(tributylstanny)thiophene afforded N,N′-bis(2-decyltetradecyl)-1,7-bis(thien-2yl)-perylene-3,4:9,10-tetracarboxylic diimide (1) in 77% yield, which was brominated by NBS to give N,N′-bis(2-decyltetradecyl)-1,7-bis(2-bromothien-5-yl)-perylene-3,4:9,10-tetracarboxylic diimide (2) in 88% yield. Then photocyclization of compound 2 in the presence of I2 yielded the monomer 3 in 64% yield. The final copolymer was synthesized through Sonogashira coupling reaction of diethynylporphyrin Zinc with the monomer 3 in a yield of 85%. The polymer has good solubility in common organic solvents such as chloroform, toluene, and THF. Molecular weight of the polymer was determined by gel permeation chromatography (GPC), using polystyrene standards as calibrants with THF as eluent. The polymer exhibits a number-average molecular weight (Mn) of 2.1 × 104, a weight-average molecular weight (Mw) of 7.8 × 104, and a polydispersity index (Mw/Mn) of 3.69. The thermal property of the polymer was determined by
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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic route to the polymer is outlined in Scheme 1. 1,7-Dibromoperylene diimide (PDI-2Br)47 and diethynylporphyrin zinc (POR)48 7824
dx.doi.org/10.1021/ma3015257 | Macromolecules 2012, 45, 7823−7828
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Previously reported copolymer poly(PDI−POR) shows two absorption bands at 454 and 912 nm in CHCl3 solution (Figure 3).43 The high-energy band of the copolymer poly(DTCDI− POR) red-shifts 14 nm relative to that of poly(PDI−POR) as a result of relatively more intense electron delocalization caused by large and planar DTCDI unit. On the other hand, the lowenergy charge-transfer band of poly(DTCDI−POR) blue-shifts 190 nm compared to that of poly(PDI−POR), due to the weaker electron-withdrawing ability of DTCDI. The low-energy band of the polymer poly(DTCDI−POR) in film red shifts 20 nm compared to that in solution due to strong electronic coupling between neighboring molecules caused by large and coplanar structure of DTPDI and porphyrin (Figure 3). The optical band gap estimated from the absorption edge in film is 1.44 eV. Electrochemistry. To estimate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy level of the conjugated copolymer, we studied electrochemical properties using cyclic voltammetry (CV) of films drop cast onto glassy carbon working electrodes. The cyclic voltammogram of the polymer is shown in Figure 4. Copolymer poly(DTCDI−POR) exhibits a
thermogravimetric analysis (TGA) under nitrogen and air. The polymer has good thermal stability with decomposition temperature (Td) at 5% weight loss of 323 °C under nitrogen and 321 °C under air (Figure 2). The Td of poly(DTCDI− POR) is lower than that (362 °C) of poly(PDI−POR) under nitrogen.43
Figure 2. TGA curves of polymer poly(DTCDI−POR) under nitrogen and air and of polymer poly(PDI−POR) under nitrogen.
One-Photon Absorption. Figure 3 shows UV−vis spectra of polymer poly(DTCDI−POR) as well as its monomers in
Figure 4. Cyclic voltammogram of poly(DTCDI−POR) and poly(PDI−POR) films in CH3CN/0.1 M [nBu4N]+[PF6]− at 50 mV s−1. The horizontal scale refers to an anodized Ag wire pseudoreference electrode.
Figure 3. UV−vis spectra of poly(DTCDI−POR) and poly(PDI− POR) in chloroform and in film as well as of the monomers POR and 3 in chloroform.
quasi reversible oxidation peak and a quasi reversible reduction wave. The HOMO and LUMO values of the polymer (Table 1) are estimated from the onset oxidation and reduction potentials, assuming the absolute energy level of FeCp2+/0 to be 4.8 eV below vacuum. The LUMO level is −3.69 eV and HOMO level is −5.29 eV. The band gap estimated from electrochemistry is 1.60 eV. The LUMO level of poly(DTCDI−POR) is similar to those (−3.7 eV) of DTCDIthiophene copolymers.45 Compared with poly(PDI−POR) which possesses LUMO level of −4.00 eV and HOMO level of −5.41 eV,43 poly(DTCDI−POR) exhibits up-shifted LUMO and HOMO levels since DTCDI is a weaker electron acceptor relative to PDI. Two-Photon Absorption. The 2PA cross sections (δ) of copolymer poly(DTCDI−POR) were measured at 1320, 1380, 1440, 1480, and 1520 nm in CHCl3 solution by using the openaperture Z-scan method. Figure 5 shows typical Z-scan traces and 2PA cross sections per repeat unit of the polymer at all these wavelengths. The polymer exhibits large 2PA cross sections up to 7809 GM/repeat unit at 1520 nm, which is a little higher than that (7704 GM at 1520 nm) of poly(PDI− POR). Although DTCDI has weaker electron withdrawing
CHCl3 solution (ca. 10−6 M). The DTCDI monomer 3 exhibits two main absorption bands with low energy λmax located at 530 nm. The DTCDI monomer exhibits well-defined vibronic features, similar to nonsubstituted PDIs, indicating enhanced πconjugation along the short molecular axis and improved planarity and rigidity of the molecular backbone.45 The porphyrin monomer POR shows typical strong Soret band at 434 nm and weak Q-band at 624 nm. In comparison with the monomers, the polymer poly(DTCDI−POR) shows a significant red shift (100 nm) of the low-energy band. Poly(DTCDI−POR) shows three absorption bands at 358 nm, 468 and 722 nm with high molar extinction coefficient (per repeat unit) of 1.05 × 105 L mol−1 cm−1 at 468 nm in chloroform. The band at ca. 358 nm can be assigned to a DTCDI-centered transition, the 468 nm intense band to overlap of DTCDI-based and porphyrin-based transition, and the broad low-energy band located at 722 nm to a transition with significant donor-to-acceptor (porphyrin to DTCDI) charge-transfer character. 7825
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Table 1. Absorption Maxima and Energy Levels of Poly(DTCDI−POR) and Poly(PDI−POR) polymer
λmaxa (nm)
λmaxb (nm)
Egc (eV)
Eoxd (V)
Eredd (V)
HOMOe (eV)
LUMOe (eV)
EgCVf (eV)
poly(DTCDI−POR) poly(PDI−POR)
468, 722 454, 912
476, 742 462, 948
1.44 1.15
0.49 0.61
−1.11 −0.80
−5.29 −5.41
−3.69 −4.00
1.60 1.41
a Absorption maxima in solution. bAbsorption maxima in film. cOptical band gap estimated from the absorption edge in film. dEox is the onset potentials vs FeCp2+/0 corresponding to oxidations, while Ered is the onset potentials vs FeCp2+/0 corresponding to reductions. eHOMO and LUMO estimated from the onset oxidation and reduction potentials, respectively, assuming the absolute energy level of FeCp2+/0 to be 4.8 eV below vacuum. f HOMO−LUMO gap estimated by electrochemistry.
Figure 5. (a) Z-scan traces of poly(DTCDI−POR) (1.21 × 10−4 M per repeat unit) in CHCl3 in a 1 mm cell at 1520 nm (I0 = 39.3 GW/cm2), 1480 nm (I0 = 47.0 GW/cm2), 1440 nm (I0 = 51.6 GW/cm2), 1380 nm (I0 = 52.4 GW/cm2), and 1320 nm (I0 = 64.1 GW/cm2) with a theoretical fit assuming a 2PA process. (b) 2PA cross sections per repeat unit of poly(DTCDI−POR) and poly(PDI−POR). Errors associated with values were determined to be ±15%.
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ability than PDI, it has better planarity, rigidity and conjugation than PDI, which is beneficial to 2PA enhancement. The previously reported analogous polymer with leadcontaining alkyne-linked porphyrins and no DTCDI units exhibited a considerably large 2PA cross section about 5200 GM/repeat unit at 1300 nm and a much smaller 2PA cross section of 650 GM/repeat unit at 1550 nm.42 The previously reported analogous polymer with zinc-containing alkyne-linked porphyrins and no DTCDI units exhibited an even smaller 2PA cross sections
